Rare earth metal oxide process including extracting rare earth metal from acidic solution with an ionic liquid composition

ABSTRACT

A method for extracting a rare earth metal from a mixture of one or more rare earth metals, said method comprising contacting an acidic solution of the rare earth metal with a composition which comprises an ionic liquid to form an aqueous phase and a non-aqueous phase into which the rare earth metal has been selectively extracted; recovering the rare earth metal from the non-aqueous phase; and processing the recovered rare earth metal into a rare earth metal oxide.

The present invention relates to the preparation of rare earth metaloxides. In particular, the present invention relates to preparation ofrare earth metal oxides using a method in which rare earth metals areextracted and separated using specifically designed ionic liquids.

Rare earth metals, which include the lanthanides (La to Lu), Y, and Sc,have unique physicochemical properties which make them crucialcomponents of numerous high-tech products and environmental technologiessuch as wind mills, LCD/LED displays, phosphors, magnet drives (harddisk), and others. These applications demand a continuous supply of highpurity rare earth metals to the industries, which is currently met bymining and processing the natural ores of these metals. However, thereare concerns that the exponentially increasing demand of these metalswill surpass the supply in coming years and therefore, it has becomeattractive to explore other secondary sources of these valuable metals.One such source is the recovery of rare earth metals from end-of-lifeand manufacturing wastes materials (often referred to as “urbanmining”), which, though quite challenging, can potentially provide acontinuous supply of the rare earth metals. One of most importantrequirements of urban mining is the development of cost effective androbust separation processes/technologies which allow selective andefficient separation of rare earth metals from each other (intra-groupseparation) to provide high purity rare earth metals.

During the last five decades various processes such as liquid-liquidextraction (e.g. Rhône-Poulenc process), ion exchange, and precipitationhave been developed. Among the various available technologies,liquid-liquid extraction has been found to be the most suitablecommercial process owing to its scalability, adaptability, andrecyclability. Additionally, the liquid-liquid extraction processes usedto date employ commercial organophosphorus extractants which do notpossess specific selectivity for individual rare earth metals, therebyleading to a number of stages to separate rare earth metals from eachother (see Table 1). Furthermore, additional processing steps aregenerally required to recover the rare earth metal in high purity. Thesefactors lead to manifold increase in processing costs thereby puttingstrain on overall costing of consumer products. Also, most employedmethods for the separation of rare earth metals necessitate the use oforganic solvents, which due to their toxicity, volatility andflammability are not considered environmentally friendly.

Some of the currently used industrial liquid-liquid extraction processesavailable for intra-group separation of rare earth metals (e.g.separation of dysprosium from neodymium) are compared in Table 1.

The separation factor for an individual rare earth metal pair isexpressed as the ratio of the distribution ratios (D_(M)) of the rareearth metals, where the distribution ratio of an individual rare earthmetal is determined as the ratio of its concentration in the non-aqueousphase to that in the aqueous phase i.e. D_(M)=[M]_(N-Aq)/[M]_(Aq). Forexample, the separation factor of Dysprosium with respect toNeodymium=D_(Dy)/D_(Nd).

TABLE 1 Comparison of the separation factors of commonly used REMextractants. Liquid- liquid Separation extraction Major component factorReference HDEHP Bis-(2-ethylhexyl)- 41.5 C. K. Gupta, N. processphosphoric acid (Dy/Nd) Krishnamurthy, Extractive Metallurgy of RareEarths, CRC, New York, 2005, pp. 1-484. Cyanex Bis-(2,4,4- 1.36 B.Swain, E.O. Otu, 272 trimethylpentyl) (Dy/Nd) Separation and processphosphinic acid Purification Technology, 83, (2011), 82-90 CyanexBis-(2,4,4- 239.3 M. Yuan, A. Luo, D. Li, 302 trimethylpentyl)- (Dy/Nd)Acta Metall. Sin. 1995, process monothiophosphinic 8, 10-14. acidSynergist 2- 1.17 N. Song, S. Tong, W. process ethylhexylphosphonic(Dy/Nd) Liu, Q.Jia, W.Zhoua and acid mono-(2- W.Liaob, J. Chem.ethylhexyl)ester; Technol. Biotechnol., sec-nonylphenoxy 2009, 84,1798-1802. acetic acid

Another of the most commonly used organophosphorous extractants, P507(2-ethylhexyl phosphoric acid mono(2-ethylhexyl) ester), also gives lowseparation factors, with the selectivity for heavy rare earth metalsgenerally being lower than for light rare earth metals (e.g. Tm/Er(3.34), Yb/Tm (3.56), and Lu/Yb (1.78)). Another significant deficiencyof many common rare earth metal extractants such as P507 is that it isdifficult to strip heavy rare earth metals completely, especially forTm(III), Yb(III), and Lu(III), even at higher acidity. Low selectivityfor rare earth metals results in too many stages required for effectiveseparation, the low extractability of rare earth metals demanding theuse of higher concentrations of the extractant. The production oforganophosphorous extractants also requires complicated syntheticprocedures starting from hazardous starting materials and the stabilityand recyclability of these extractants is limited. Emulsification andleaching of extractants has been identified as another common problem.

A chelating diamide extractant attached to a silica support was reportedby Fryxell et al. for the separation of lanthanides (Inorganic ChemistryCommunications, 2011, 14, 971-974). However, this system was unable toextract rare earth metals under acidic conditions (pH<5) and cruciallyshowed very low uptake and separation factors between rare earth metals.

Ionic liquids have also been used as potential extractants for rareearth metals. Binnemans et al. reported the extraction of Nd and Dy or Yand Eu from mixtures of transition metal compounds with a betainiumbis(trifluoromethyl-sulfonyl)imide ionic liquid (Green Chemistry, 2015,17, 2150-2163; Green Chemistry, 2015, 17, 856-868). However, this systemwas unable to selectively perform intra-group separation between rareearth metals.

Chai et al. reported the use of an ionic liquid based on 2-ethylhexylphosphonic acid mono(2-ethylhexyl) ester (P507) with atrioctylmethylammonium cation for separation of rare earth metals(Hydrometallurgy, 2015, 157(C), 256-260). In this case only lowdistribution factors and separation factors were observed, indicating alack of extractability and selectivity. In addition, during recovery ofthe rare earth metal from the ionic liquid, the acid added willdecompose the acid-base pair ionic liquid, which must then beregenerated by metathesis.

Separation of Nd and Dy was reported by Schelter et al., wherebyseparation was achieved by precipitation using a tripodal nitroxideligand to form Nd and Dy complexes with differing solubilities inbenzene. However, precipitation is not considered to be a commerciallyviable process and, in addition, the process requires the use ofspecific rare earth metal precursors and an inert, moisture-freeenvironment, which is highly impractical for commercial scale up. Thismethod also relies on the use of benzene to achieve high separation,which is a very toxic solvent.

Therefore, there is a need for the development of effective processesthat enhance separation selectivity and extractability, whilstminimizing environmental pollution.

Moreover, once a rare earth metal has been separated, it is necessary toprovide it in a suitable form for storage, transportation and use.Accordingly, there is also a need for the development of processes thatenhance separation selectivity and extractability, whilst also providingthe rare earth metal in a useful form.

By using an ionic liquid having a cation comprising particular features,it has been found that rare earth metals may be extracted and separatedfrom each other with increased selectivity and extractability incomparison to known methods using different extractants and readilyfurther processed into a stable oxide form. As the method uses an ionicliquid, the extractant can also provide decreased volatility andflammability, potentially leading to safer and more environmentallyfriendly rare earth metal extraction.

Thus, in a first aspect, the present invention provides a method forpreparing a rare earth metal oxide from a mixture of one or more rareearth metals, said method comprising:

-   -   contacting an acidic solution of the rare earth metal with a        composition which comprises an ionic liquid to form an aqueous        phase and a non-aqueous phase into which the rare earth metal        has been selectively extracted,    -   recovering the rare earth metal from the non-aqueous phase; and    -   processing the recovered rare earth metal into a rare earth        metal oxide,        wherein the ionic liquid has the formula:        [Cat⁺][X⁻]        in which:    -   [Cat⁺] represents a cationic species having the structure:

-   -   where: [Y⁺] comprises a group selected from ammonium,        benzimidazolium, benzofuranium, benzothiophenium,        benzotriazolium, borolium, cinnolinium, diazabicyclodecenium,        diazabicyclononenium, 1,4-diazabicyclo[2.2.2]octanium,        diazabicyclo-undecenium, dithiazolium, furanium, guanidinium,        imidazolium, indazolium, indolinium, indolium, morpholinium,        oxaborolium, oxaphospholium, oxazinium, oxazolium,        iso-oxazolium, oxothiazolium, phospholium, phosphonium,        phthalazinium, piperazinium, piperidinium, pyranium, pyrazinium,        pyrazolium, pyridazinium, pyridinium, pyrimidinium,        pyrrolidinium, pyrrolium, quinazolinium, quinolinium,        iso-quinolinium, quinoxalinium, quinuclidinium, selenazolium,        sulfonium, tetrazolium, thiadiazolium, iso-thiadiazolium,        thiazinium, thiazolium, iso-thiazolium, thiophenium, thiuronium,        triazinium, triazolium, iso-triazolium and uronium groups;        -   each EDG represents an electron donating group; and        -   L₁ represents a linking group selected from C₁₋₁₀            alkanediyl, C₂₋₁₀ alkenediyl, C₁₋₁₀ dialkanylether and C₁₋₁₀            dialkanylketone groups;    -   each L₂ represents a linking group independently selected from        C₁₋₂ alkanediyl, C₂ alkenediyl, C₁₋₂ dialkanylether and C₁₋₂        dialkanylketone groups; and    -   [X⁻] represents an anionic species.

The term “ionic liquid” as used herein refers to a liquid that iscapable of being produced by melting a salt, and when so producedconsists solely of ions. An ionic liquid may be formed from ahomogeneous substance comprising one species of cation and one speciesof anion, or it can be composed of more than one species of cationand/or more than one species of anion. Thus, an ionic liquid may becomposed of more than one species of cation and one species of anion. Anionic liquid may further be composed of one species of cation, and oneor more species of anion. Still further, an ionic liquid may be composedof more than one species of cation and more than one species of anion.

The term “ionic liquid” includes compounds having both high meltingpoints and compounds having low melting points, e.g. at or below roomtemperature. Thus, many ionic liquids have melting points below 200° C.,particularly below 100° C., around room temperature (15 to 30° C.), oreven below 0° C. Ionic liquids having melting points below around 30° C.are commonly referred to as “room temperature ionic liquids” and areoften derived from organic salts having nitrogen-containing heterocycliccations. In room temperature ionic liquids, the structures of the cationand anion prevent the formation of an ordered crystalline structure andtherefore the salt is liquid at room temperature.

Ionic liquids are most widely known as solvents. Many ionic liquids havebeen shown to have negligible vapour pressure, temperature stability,low flammability and recyclability. Due to the vast number ofanion/cation combinations that are available it is possible to fine-tunethe physical properties of the ionic liquid (e.g. melting point,density, viscosity, and miscibility with water or organic solvents) tosuit the requirements of a particular application.

Typically, when rare earth metals are extracted from sources such asores or waste materials, the resulting product is a mixture of rareearth metals dissolved in an aqueous acidic solution. In the methodaccording to the present invention, rare earth metals may be selectivelyextracted directly from an aqueous acidic feed, negating the need toapply significant processing to the feed prior to extraction.

It will be appreciated that in order to form an aqueous phase and anon-aqueous phase when contacted with the acidic solution, thecomposition comprising an ionic liquid will be sufficiently hydrophobicsuch that a phase separation will occur between the aqueous solution andthe composition.

By the use of the composition comprising an ionic liquid as definedherein, it has been surprisingly found that increased selectivity andextractability may be obtained in the extraction of rare earth metalsfrom an acidic solution. The combination of high extractability(indicated by distribution ratio) and selectivity (indicated byseparation factors) is key to a commercially effective separationprocess because the number of separation stages necessary to produce aproduct may be reduced without sacrificing purity. For example,according to the method of the present invention, mixtures of dysprosiumand neodymium may be separated with a selectivity (separation factor) ofover 1000:1 in a single contact. This represents a substantial increaseover known systems as reported in Table 1.

Without wishing to be bound by any particular theory, it is believedthat the presence of the central nitrogen donor atom in the ionic liquidallows for differing binding strengths to different rare earth metals asa result of differing ionic radii due to lanthanide contraction. In thisway, some rare earth metals are preferentially bound by the hydrophobicionic liquid extractant, which results in effective intra-groupseparation of the rare earth metals. It is believed that the arrangementof this variable nitrogen binding as part of an ionic liquid providesthe particularly effective extraction of rare earth metals describedherein. Nonetheless, it will be appreciated that the ionic liquidcomprising a nitrogen donor, whilst discriminating between differentrare earth metals, must have additional electron donating groupsappended in order to provide sufficient extractability.

Once the rare earth metal has been extracted into the ionic liquid, therare earth metal is recovered from the non-aqueous phase. This recoverymay be performed using any suitable means, however it is preferred thatthe rare earth metal is recovered from the non-aqueous phase bystripping with an acidic stripping solution. This gives an acidicstripping solution comprising the recovered rare earth metal.

It will be appreciated that the acidic stripping solution may be anyacidic solution which liberates the rare earth metal from the ionicliquid. In most embodiments, the acidic stripping solution will be anaqueous acidic stripping solution and the acid will substantially remainin the aqueous phase on contact with the ionic liquid. Preferably, theacidic stripping solution comprises an aqueous hydrochloric acid ornitric acid solution.

The stripping of the rare earth metal may be conducted in any suitablemanner. Preferably, the ionic liquid is contacted with an acidicstripping solution for 2 or more stripping cycles to completely stripthe rare earth metal, more preferably 2 or 3 stripping cycles are used.In some embodiments, a single stripping cycle may be used. A “strippingcycle” as referred to herein will typically comprise contacting theacidic stripping solution with the composition, equilibrating for anamount of time, for example 5 to 30 minutes, and separating the aqueousand organic phases. A second cycle may be conducted by contacting thecomposition with another acidic stripping solution substantially free ofrare earth metals.

One advantage of the ionic liquid extractant as described in relation tothe first aspect is that the rare earth metal may be stripped from theionic liquid at a relatively high pH. This saves costs associated withboth the amount and the strength of acid needed to strip the rare earthmetals from the ionic liquid and the equipment necessary to handle suchstrong acids. In addition, it is possible to completely strip rare earthmetals from the ionic liquid at a relatively high pH, whilst for manyknown extractants such as P507 it is difficult to completely strip heavyrare earth metals (e.g. Tm(III), Yb(III), Lu(III)) even at low pH.

Thus, the acidic stripping solution preferably has a pH of 0 or higher.In preferred embodiments, the acidic stripping solution has a pH of 1 orlower.

The acidic stripping solution comprising the recovered rare earth metalmay be processed into a rare earth metal by:

-   -   contacting the acidic stripping solution comprising the        recovered rare earth metal with oxalic acid to give a rare earth        metal oxalate; and    -   converting, e.g. by calcination, the rare earth metal oxalate        into the rare earth metal oxide.

The acidic stripping solution comprising the recovered rare earth metalis preferably contacted with oxalic acid at room temperature, e.g. at atemperature of from 15 to 30° C. For instance, the contacting may becarried out without the application of heat or cooling.

The contacting is preferably carried out at ambient pressure (e.g. at apressure of approximately 100 kPa). For instance, the contacting may becarried out without the application of pressure.

The acidic stripping solution comprising the recovered rare earth metaland the oxalic acid are preferably mixed, e.g. by being stirred.

The oxalic acid may be used in an amount which is sufficient toprecipitate the rare earth metal as a rare earth metal oxalate, e.g. ina molar excess as compared to the rare earth metal. The method of thepresent invention preferably comprises separating the precipitated rareearth metal oxalate from the acidic stripping solution, e.g. byfiltration or other known solid-liquid separation techniques, before itis converted into its oxide form. The separated solid may be washed,e.g. with water.

The rare earth metal oxalate may be calcined at a temperature of atleast 500° C., preferably at least 700° C., and more preferably at least900° C. The rare earth metal oxalate may be calcined at a temperature ofup to 1500° C., preferably up to 1300° C., and more preferably up to1100° C. Thus, the rare earth metal oxalate may be calcined at atemperature of from 500 to 1500° C., preferably from 700 to 1300° C.,and more preferably from 900 to 1100° C.

The calcination is preferably carried out at ambient pressure (e.g. at apressure of approximately 100 kPa). For instance, the calcination may becarried out without the application of pressure.

Calcination may take place in an oxygen atmosphere or in air.

Alternatively, the rare earth metal may be recovered from thenon-aqueous phase by electrodeposition. The electrodeposited rare earthmetal may then be processed into a rare earth metal oxide, optionally byconversion first into a hydroxide or nitrate.

The rare earth metal oxides will typically have the formula: M₂O₃, whereM is the rare earth metal. The rare earth metal oxide will typically beprepared in a powder form.

The oxide form of rare earth metals is advantageously stable, enablingthe rare earth metal to be, among other things, stored, transported orused. For instance, the rare earth metal oxide may be further processedinto an oxide-derived material. One use for the rare earth metal oxideis in a magnet. Thus, the method of the present invention may furthercomprise processing the rare earth metal oxide into a magnet. Inembodiments where the source of the rare earth metal is also a magnet,the method of the present invention therefore represents a completerecycling process.

In some instances, it may be desirable to provide a rare earth metaloxalate rather than a rare earth metal oxide. Thus, the presentinvention may provide a method for preparing a rare earth metal oxalatefrom a mixture of one or more rare earth metals, the method comprising:contacting (as described herein) an acidic solution of the rare earthmetal with a composition which comprises an ionic liquid to form anaqueous phase and a non-aqueous phase into which the rare earth metalhas been selectively extracted; recovering (as described herein) therare earth metal from the non-aqueous phase; and processing (asdescribed herein) the recovered rare earth metal into a rare earth metaloxalate.

In preferred embodiments, the method comprises extracting a rare earthmetal from a mixture of two or more rare earth metals. Preferably, theacidic solution comprises a first and a second rare earth metal, and themethod comprises:

-   -   (a) preferentially partitioning the first rare earth metal into        the non-aqueous phase, recovering the first rare earth metal        from the non-aqueous phase, and processing the recovered first        rare earth metal into a rare earth metal oxide.

It will be appreciated that the recovering and processing steps arepreferably as described above.

Preferably, the method further comprises, in step (a), separating thenon-aqueous phase from the acidic solution; and

-   -   (b) contacting the acidic solution depleted of the first rare        earth metal with the composition which comprises an ionic        liquid, and optionally recovering the second rare earth metal        therefrom.

The method preferably comprises recovering the second rare earth metalfrom the non-aqueous phase and processing the recovered second rareearth metal into a rare earth metal oxide. The recovering and processingsteps are preferably carried out as described above.

In some preferred embodiments the first rare earth metal is recoveredfrom the non-aqueous phase in step (a), and said non-aqueous phase isrecycled and used as the composition in step (b).

It will be appreciated that, because the extractability (distributionfactor) for a particular rare earth metal varies with pH, it may bepreferred to extract different rare earth metals at different pH levels.For example, the acidic solution may have a lower pH in step (a) incomparison to that in step (b). Preferably, the acidic solution has a pHof less than 3.5 in step (a), and the acidic solution has a pH ofgreater than 3.5 in step (b). Typically, 2 or 3 extraction cycles willbe performed at a particular pH. Although the above embodiment describesextraction in only two different pH values, it will be appreciated thata separation of rare earth metals will usually be conducted across arange of pH values, with a gradual increase in pH and multipleextraction steps. For example, where three or more rare earth metals areseparated, several separation steps may be conducted in across aparticular pH range, for example from pH 1 to 4.

The acidic solution from which the rare earth metal is extracted mayhave any suitable pH. Preferably, the rare earth metal is extracted at apH of more than 1, more preferably at a pH of from 2 to 4.

The pH level of the acidic solution of the rare earth metal may beadjusted in any suitable way, as is well known to those skilled in theart. For example, the pH level of the acidic solution may be altered bythe addition of acid scavengers such as mildly alkaline solutionsincluding sodium carbonate, sodium bicarbonate, ammonia, CO₂, amines oralcohols.

The above embodiments refer to the separation of a particular rare earthmetal from another directly from the acidic solution of the rare earthmetal at varying pH levels. However, it will be understood that anysuitable extraction sequence may be used to separate rare earth metals.For example, two or more rare earth metals may be extracted from theacidic solution to the non-aqueous phase simultaneously at a higher pH,followed by back-extraction of the non-aqueous phase with acidicsolutions having a lower pH to separate individual rare earth metals.Thus, all or only some of the rare earth metals present in the acidicsolution may initially be extracted from the acidic solution using thecomposition comprising the ionic liquid.

It will be appreciated that the separation of certain pairs of rareearth metals are of particular importance due to their simultaneousrecovery from valuable waste materials. For example, Nd and Dy arewidely used in permanent magnets for numerous applications such as harddisks, MRI scanners, electric motors and generators. La and Eu are alsoan important pair due to their common use in lamp phosphors, otherphosphors include Y and Eu (YOX phosphors); La, Ce and Tb (LAPphosphors); Gd, Ce and Tb (CBT phosphors); and Ce, Tb (CAT phosphors).

Thus, in preferred embodiments, the first rare earth metal isdysprosium, and the second rare earth metal is neodymium. In otherpreferred embodiments, the first rare earth metal is europium, and thesecond rare earth metal is lanthanum. In yet other preferredembodiments, the first rare earth metal is terbium, and the second rareearth metal is cerium.

The composition may be contacted with the acidic solution in anysuitable manner and in any suitable ratio such that exchange of rareearth metals is achieved between the aqueous and non-aqueous phases.

The composition is preferably added to the acidic solution in a volumeratio of from 0.5:1 to 2:1, preferably 0.7:1 to 1.5:1, more preferably0.8:1 to 1.2:1, for example 1:1. Nonetheless, it will be appreciatedthat the volume ratio will vary depending on the manner in which theacidic solution is contacted with the composition comprising the ionicliquid.

Preferably, prior to contacting the composition with the acidic solutionof the rare earth metal the composition is equilibrated with an acidicsolution having the same pH as the acidic solution of the rare earthmetal. In this way, the mixture of the composition and the acidicsolution will generally remain at the desired pH level during theextraction.

The composition may be contacted with the acidic solution of the rareearth metal under any conditions suitable for extracting the rare earthmetal.

It will be appreciated that the temperature employed during contactingof the acidic solution with the composition comprising the ionic liquidmay be any suitable temperature and may vary according to the viscosityof the composition comprising the ionic liquid. For example, where ahigher viscosity composition is used, a higher temperature may benecessary in order to obtain optimal results.

Preferably, the acidic solution is contacted with the composition atambient temperature, i.e. without external heating or cooling. It willnonetheless be appreciated that temperature changes may naturally occurduring the extraction as a result of contacting the composition with theacidic solution.

The composition may be contacted with the acidic solution of the rareearth metal for any length of time suitable to facilitate extraction ofthe rare earth metal into the non-aqueous phase. Preferably, the lengthof time will be such that an equilibrium is reached and the proportionsof rare earth metal in the aqueous and non-aqueous phases are constant.In preferred embodiments, the method comprises contacting the acidicsolution of the rare earth metal and the composition for from 1 to 40minutes, preferably from 5 to 30 minutes.

Preferably, the method comprises contacting and physically mixing theacidic solution of the rare earth metal and the composition. Such mixingwill usually speed up extraction of the rare earth metal. Any suitableapparatus may be used to achieve this and mixing apparatus is well knownin the art. For example, the mixture may be mixed using an agitator orstirrer. The mixing apparatus may comprise equipment specificallydesigned for multi-phase mixing such as high shear devices.Alternatively, mixing may comprise shaking the mixture, for example,using a wrist action shaker.

The separation of the aqueous and non-aqueous phases may be performed byany suitable method, for example by use of small scale apparatus such asa separating funnel or Craig apparatus. It will be appreciated that thephases will normally be allowed to settle prior to separation. Settlingmay be under gravity or preferably accelerated by the use of additionalequipment such as centrifuge. Alternatively, aqueous and non-aqueousphases may be separated by the use of equipment which both contacts andseparates the phases, for example a centrifugal extractor, a pulsedcolumn, or a combined mixer-settler.

It will be understood that in order to extract or separate some rareearth metals, multiple extractions and separations may be performed.This may involve multiple extractions of the acidic solution of the rareearth metal with the composition or multiple back-extractions of thenon-aqueous phase with an aqueous acidic solution. In accordance withthe present invention, fewer steps are required to separate rare earthmetals due to the ionic liquid extractant giving separation factors anddistribution ratios above those typically found in previous systems.

The term electron donating group (EDG) as used herein will be understoodto include any group having a pair of electrons available to form acoordinate bond with an acceptor. In particular, it will be appreciatedthat an electron donating group, as defined herein, refers to groupshaving an available pair of electrons able to coordinate to a rare earthmetal to form a metal-ligand complex. It will also be understood thatthe EDGs will typically have a single atom from which the electrons aredonated to form a bond. However, electrons may alternatively be donatedfrom one or more bonds between atoms, i.e. EDG may represent a ligandwith a hapticity of 2 or more.

It will be understood that the arrangement of the EDGs and the linkersL₂ will be such that the EDGs and the central nitrogen atom are able tocoordinate to a rare earth metal simultaneously.

Preferably, when the nitrogen linking L₁ to each L₂ and one of the EDGboth coordinate to a metal, the ring formed by the nitrogen, L₂, the EDGand the metal is a 5 or 6 membered ring, preferably a 5 membered ring.

In preferred embodiments, [Y⁺] represents an acyclic cation selectedfrom:[—N(R^(a))(R^(b))(R^(c))]⁺, [—P(R^(a))(R^(b))(R^(c))]⁺ and[—S(R^(a))(R^(b))]⁺,

-   -   wherein: R^(a), R^(b) and R^(c) are each independently selected        from optionally substituted C₁₋₃₀ alkyl, C₃₋₈ cycloalkyl and        C₆₋₁₀ aryl groups.

In other preferred embodiments, [Y⁺] represents a cyclic cation selectedfrom:

-   -   wherein: R^(a), R^(b), R^(c), R^(d), R^(e) and R^(f) are each        independently selected from: hydrogen and optionally substituted        C₁₋₃₀ alkyl, C₃₋₈ cycloalkyl and C₆₋₁₀ aryl groups, or any two        of R^(a), R^(b), R^(c), R^(d) and R^(e) attached to adjacent        carbon atoms form an optionally substituted methylene chain        —(CH₂)_(q)— where q is from 3 to 6.

Suitably, in preferred embodiments, at least one of R^(a), R^(b), R^(c),R^(d), R^(e) and R^(f) is a C₁₋₅ alkyl group substituted with —CO₂R^(x),—OC(O)R^(x), —CS₂R^(x), —SC(S)R^(x), —S(O)OR^(x), —OS(O)R^(x),—NR^(x)C(O)NR^(y)R^(z), —NR^(x)C(O)OR^(y), —OC(O)NR^(y)R^(z),—NR^(x)C(S)OR^(y), —OC(S)NR^(y)R^(z), —NR^(x)C(S)SR^(y),—SC(S)NR^(y)R^(z), —NR^(x)C(S)NR^(y)R^(z), —C(O)NR^(y)R^(z),—C(S)NR^(y)R^(z), wherein R^(x), R^(y) and R^(z) are independentlyselected from hydrogen or C₁₋₆ alkyl.

In another preferred embodiment of the invention, [Y⁺] represents asaturated heterocyclic cation selected from cyclic ammonium,1,4-diazabicyclo[2.2.2]octanium, morpholinium, cyclic phosphonium,piperazinium, piperidinium, quinuclidinium, and cyclic sulfonium.

Preferably, [Y⁺] represents a saturated heterocyclic cation having theformula:

-   -   wherein: R^(a), R^(b), R^(c), R^(d), R^(e) and R^(f), are as        defined above.

Preferably, at least one of R^(a), R^(b), R^(c), R^(d), R^(e) and R^(f)is C₁₋₃ alkyl group substituted with —CO₂R^(x), —C(O)NR^(y)R^(z),wherein R^(x), R^(y) and R^(z) are each independently selected from C₃₋₆alkyl.

More preferably, at least one of R^(a), R^(b), R^(c), R^(d), R^(e) andR^(f) represents a group selected from:

-   -   wherein R^(y)=R^(z), and wherein R^(x), R^(y) and R^(z) are each        selected from C₃₋₆ alkyl, preferably C₄ alkyl, for example i-Bu.

Yet more preferably, at least one of R^(a), R^(b), R^(c), R^(d), R^(e)and R^(f) represents a group selected from:

-   -   wherein R^(y)=R^(z), and wherein R^(y) and R^(z) are selected        from C₃₋₆ alkyl, preferably C₄ alkyl, for example i-Bu.

In preferred embodiments, one of R^(a), R^(b), R^(c), R^(d), R^(e) andR^(f) is a substituted C₁₋₅ alkyl group, and the remainder of R^(a),R^(b), R^(c), R^(d), R^(e) and R^(f) are independently selected from Hand unsubstituted C₁₋₅ alkyl groups, preferably the remainder of R^(a),R^(b), R^(c), R^(d), R^(e) and R^(f) are H.

Preferably, [Y⁺] represents a cyclic cation selected from:

more preferably [Y⁺] represents the cyclic cation:

preferably wherein R^(f) is a substituted C₁₋₆ alkyl group, and theremainder of R^(a), R^(b), R^(c), R^(d), R^(e) and R^(f) areindependently selected from H and unsubstituted C₁₋₆ alkyl groups.

In preferred embodiments, L₁ represents a linking group selected fromC₁₋₁₀ alkanediyl and C₁₋₁₀ alkenediyl groups, more preferably selectedfrom C₁₋₅ alkanediyl and C₂₋₅ alkenediyl groups, and most preferablyselected from C₁₋₅ alkanediyl groups, for example a linking groupselected from —CH₂—, —C₂H₄— and —C₃H₆—.

In preferred embodiments, each L₂ represents a linking groupindependently selected from C₁₋₂ alkanediyl and C₂ alkenediyl groups,preferably selected from C₁₋₂ alkanediyl groups, for exampleindependently selected from —CH₂— and —C₂H₄—.

Each EDG may be any suitable electron donating group able to form acoordinate bond with a rare earth metal to form a metal-ligand complex.

Preferably, each EDG represents an electron donating group independentlyselected from ——CO₂R^(x), —OC(O)R^(x), —CS₂R^(x), —SC(S)R^(x),—S(O)OR^(x), —OS(O)R^(x), —NR^(x)C(O)NR^(y)R^(z), —NR^(x)C(O)OR^(y),—OC(O)NR^(y)R^(z), —NR^(x)C(S)OR^(y), —OC(S)NR^(y)R^(z),—NR^(x)C(S)SR^(y), —SC(S)NR^(y)R^(z), —NR^(x)C(S)NR^(y)R^(z),—C(O)NR^(y)R^(z), —C(S)NR^(y)R^(z), wherein R^(x), R^(y) and R^(z) areindependently selected from H or C₁₋₆ alkyl. More preferably, each EDGrepresents an electron donating group independently selected from—CO₂R^(x) and —C(O)NR^(y)R^(z), wherein R^(x), R^(y) and R^(z) are eachindependently selected from C₃₋₆ alkyl.

In preferred embodiments, each -L₂-EDG represents an electron donatinggroup independently selected from:

-   -   wherein R^(y)=R^(z), and wherein R^(x), R^(y) and R^(z) are each        selected from C₃₋₆ alkyl, preferably C₄ alkyl, for example i-Bu.

More preferably, each -L₂-EDG represents an electron donating groupindependently selected from:

-   -   wherein R^(y)=R^(z), and wherein R^(y) and R^(z) are selected        from C₃₋₆ alkyl, preferably C₄ alkyl, for example i-Bu.

It will be appreciated that, as set out previously, the extraction ofrare earth metals is provided by the specific functionality of thecation of the ionic liquid. Thus, any suitable anionic species [X⁻] maybe used as part of the ionic liquid used in the method of the presentinvention.

Preferably, [X⁻] represents one or more anionic species selected from:hydroxides, halides, perhalides, pseudohalides, sulphates, sulphites,sulfonates, sulfonimides, phosphates, phosphites, phosphonates,phosphinates, methides, borates, carboxylates, azolates, carbonates,carbamates, thiophosphates, thiocarboxylates, thiocarbamates,thiocarbonates, xanthates, thiosulfonates, thiosulfates, nitrate,nitrite, tetrafluoroborate, hexafluorophosphate and perchlorate,halometallates, amino acids, borates, polyfluoroalkoxyaluminates.

For example, [X⁻] preferably represents one or more anionic speciesselected from:

-   -   a) a halide anion selected from: F⁻, Cl⁻ or, Br⁻, I⁻;    -   b) a perhalide anion selected from: [I₃]⁻, [I₂Br]⁻, [IBr₂]⁻,        [Br₃]⁻, [Br₂C]⁻, [BrCl₂]⁻, [ICl₂]⁻, [I₂Cl]⁻, [Cl₃]⁻;    -   c) a pseudohalide anion selected from: [N₃]⁻, [NCS]⁻, [NCSe]⁻,        [NCO]⁻, [CN]⁻;    -   d) a sulphate anion selected from: [HSO₄]⁻, [SO₄]²⁻,        [R²OSO₂O₂]⁻;    -   e) a sulphite anion selected from: [HSO₃]⁻, [SO₃]²⁻, [R²OSO₂]⁻;    -   f) a sulfonate anion selected from: [R¹SO₂O]⁻;    -   g) a sulfonimide anion selected from: [(R¹SO₂)₂N]⁻;    -   h) a phosphate anion selected from: [H₂PO₄]⁻, [HPO₄]²⁻, [PO₄]³⁻,        [R²OPO₃]²⁻, [(R²O)₂PC₂]⁻;    -   i) a phosphite anion selected from: [H₂PO₃]⁻, [HPO₃]²⁻,        [R²OPO₂]²⁻, [(R²O)₂PO]⁻;    -   j) a phosphonate anion selected from: [R¹PO₃]²⁻,        [R¹P(O)(OR²)O]⁻;    -   k) a phosphinate anion selected from: [R¹R²P(O)O]—;    -   l) a methide anion selected from: [(R¹SO₂)₃C]⁻;    -   m) a borate anion selected from: [bisoxalatoborate],        [bismalonatoborate]        tetrakis[3,5-bis(trifluoromethyl)phenyl]borate,        tetrakis(pentafluorophenyl)borate;    -   n) a carboxylate anion selected from: [R²CO₂]⁻;    -   o) an azolate anion selected from:        [3,5-dinitro-1,2,4-triazolate], [4-nitro-1,2,3-triazolate],        [2,4-dinitroimidazolate], [4,5-dinitroimidazolate],        [4,5-dicyano-imidazolate], [4-nitroimidazolate], [tetrazolate];    -   p) a sulfur-containing anion selected from: thiocarbonates (e.g.        [R²OCS₂]⁻, thiocarbamates (e.g. [R² ₂NCS₂]⁻), thiocarboxylates        (e.g. [R¹CS₂]⁻), thiophosphates (e.g. [(R²O)₂PS₂]⁻),        thiosulfonates (e.g. [RS(O)₂S]⁻), thiosulfates (e.g.        [ROS(O)₂S]⁻);    -   q) a nitrate ([NO₃]⁻) or nitrite ([NO₂]⁻) anion;    -   r) a tetrafluoroborate ([BF₄ ⁻]), hexafluorophosphate ([PF₆ ⁻]),        hexfluoroantimonate ([SbF₆ ⁻]) or perchlorate ([ClO₄ ⁻]) anion;    -   s) a carbonate anion selected from [CO₃]²⁻, [HCO₃]⁻, [R²CO₃]⁻;        preferably [MeCO₃]⁻;    -   t) polyfluoroalkoxyaluminate anions selected from [Al(OR^(F))₄        ⁻], wherein R^(F) is selected from C₁₋₆ alkyl substituted by one        or more fluoro groups;    -   where: R¹ and R² are independently selected from the group        consisting of C₁-C₁₀ alkyl, C₆ aryl, C₁-C₁₀ alkyl(C₆)aryl and C₆        aryl(C₁-C₁₀)alkyl each of which may be substituted by one or        more groups selected from: fluoro, chloro, bromo, iodo, C₁ to C₆        alkoxy, C₂ to C₁₂ alkoxyalkoxy, C₃ to C₈ cycloalkyl, C₆ to C₁₀        aryl, C₇ to C₁₀ alkaryl, C₇ to C₁₀ aralkyl, —CN, —OH, —SH, —NO₂,        —CO⁻²R^(x), —OC(O)R^(x), —C(O)R^(x), —C(S)R^(x), —CS₂R^(x),        —SC(S)R^(x), —S(O)(C₁ to C₆)alkyl, —S(O)O(C₁ to C₆)alkyl,        —OS(O)(C₁ to C₆)alkyl, —S(C₁ to C₆)alkyl, —S—S(C₁ to C₆ alkyl),        —NR^(x)C(O)NR^(y)R^(z), —NR^(x)C(O)OR^(y), —OC(O)NR^(y)R^(z),        —NR^(x)C(S)O R^(y), —OC(S)NR^(y)R^(z), —NR^(x)C(S)SR^(y),        —SC(S)NR^(y)R^(z), —NR^(x)C(S)NR^(y)R^(z), —C(O)NR^(y)R^(z),        —C(S)NR^(y)R^(z), —NR^(y)R^(z), or a heterocyclic group, wherein        R^(x), R^(y) and R^(z) are independently selected from hydrogen        or C₁ to C₆ alkyl, wherein R¹ may also be fluorine, chlorine,        bromine or iodine.

While [X⁻] may be any suitable anion, it is preferred that [X⁻]represents a non-coordinating anion. The term “non-coordinating anion”used herein, which is common in the field of ionic liquids and metalcoordination chemistry, is intended to mean an anion that does notcoordinate with a metal atom or ion, or does so only weakly. Typically,non-coordinating anions have their charge dispersed over several atomsin the molecule which significantly limits their coordinating capacity.This limits the effect interference of the anion with the selectivecoordination of the cation [Cat⁺] with the rare earth metal.

Thus, more preferably, [X⁻] represents one or more non-coordinatinganionic species selected from: bistriflimide, triflate,bis(alkyl)phosphinates such as bis(2,4,4-trimethylpentyl)phosphinate,tosylate, perchlorate, [Al(OC(CF₃)₃)₄ ⁻],tetrakis[3,5-bis(trifluoromethyl)phenyl]borate,tetrakis(pentafluorophenyl)borate, tetrafluoroborate,hexfluoroantimonate and hexafluorophosphate anions; and most preferablyfrom bistriflimide, triflate and bis(2,4,4-trimethylpentyl)phosphinateanions. Phosphinate anions in particular have been shown to give highlevels of extractability.

In some preferred embodiments, [Cat⁺] represents one or more ionicspecies having the structure:

-   -   where: [Z⁺] represents a group selected from ammonium,        benzimidazolium, benzofuranium, benzothiophenium,        benzotriazolium, borolium, cinnolinium, diazabicyclodecenium,        diazabicyclononenium, 1,4-diazabicyclo[2.2.2]octanium,        diazabicyclo-undecenium, dithiazolium, furanium, guanidinium,        imidazolium, indazolium, indolinium, indolium, morpholinium,        oxaborolium, oxaphospholium, oxazinium, oxazolium,        iso-oxazolium, oxothiazolium, phospholium, phosphonium,        phthalazinium, piperazinium, piperidinium, pyranium, pyrazinium,        pyrazolium, pyridazinium, pyridinium, pyrimidinium,        pyrrolidinium, pyrrolium, quinazolinium, quinolinium,        iso-quinolinium, quinoxalinium, quinuclidinium, selenazolium,        sulfonium, tetrazolium, thiadiazolium, iso-thiadiazolium,        thiazinium, thiazolium, iso-thiazolium, thiophenium, thiuronium,        triazinium, triazolium, iso-triazolium and uronium groups.

It will be understood that the composition may comprise the ionic liquidas defined above in combination with a diluent. Typically, a diluent maybe used in order to decrease the viscosity of the composition where theionic liquid has a high viscosity, which limits its practical use inliquid-liquid extraction. A diluent may also be used to save costs wherethe diluent is cheaper to produce than the ionic liquid. It will beunderstood that any diluent added to the composition will besufficiently hydrophobic so as to allow the separation of thecomposition and the acidic solution of the rare earth metal into anaqueous and non-aqueous phase. In some embodiments, the diluent mayenhance the hydrophobicity of the composition.

Thus, in preferred embodiments, the composition further comprises alower viscosity ionic liquid. The term “lower viscosity ionic liquid”will be understood to mean that this ionic liquid has a lower viscositythan the ionic liquid extractant described previously. As mentioned, itwill be understood that the lower viscosity ionic liquid will besufficiently hydrophobic so as to allow the separation of thecomposition and the acidic solution of the rare earth metal into anaqueous and non-aqueous phase. It will also be appreciated that thehydrophobicity may be provided by either of the cation or anion of thelower viscosity ionic liquid, or by both.

By the use of an ionic liquid as a diluent, the decreased volatility andflammability offered by the ionic liquid extractant may be maintained togive a potentially safer and more environmentally friendly rare earthmetal extraction process.

In preferred embodiments, the cation of the lower viscosity ionic liquidis selected from ammonium, benzimidazolium, benzofuranium,benzothiophenium, benzotriazolium, borolium, cinnolinium,diazabicyclodecenium, diazabicyclononenium,1,4-diazabicyclo[2.2.2]octanium, diazabicyclo-undecenium, dithiazolium,furanium, guanidinium, imidazolium, indazolium, indolinium, indolium,morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium,iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium,piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium,pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium,quinazolinium, quinolinium, iso-quinolinium, quinoxalinium,quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium,iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium,thiuronium, triazinium, triazolium, iso-triazolium and uronium groups.

Preferably the cation of the lower viscosity ionic liquid is selectedfrom phosphonium, imidazolium and ammonium groups.

In some preferred embodiments, the cation of the lower viscosity ionicliquid is selected from:[N(R³)(R⁴)(R⁵)(R⁶)]⁺ and [P(R³)(R⁴)(R⁵)(R⁶)]⁺,

-   -   wherein: R³, R⁴, R⁵ and R⁶ are each independently selected from        optionally substituted C₁₋₂₀ alkyl, C₃₋₈ cycloalkyl and C₆₋₁₀        aryl groups.

In more preferred embodiments, the cation of the lower viscosity ionicliquid is [P(R³)(R⁴)(R⁵)(R⁶)]⁺, wherein R³, R⁴, R⁵ are selected fromC₁₋₁₀ alkyl, preferably C₂₋₆ alkyl, and R⁶ is selected from C₄₋₂₀ alkyl,preferably C₈₋₁₄ alkyl. For example, the cation of the lower viscosityionic liquid may be selected from triethyloctyl phosphonium (P₂₂₂₍₈₎]⁺),tributyloctyl phosphonium (P₄₄₄₍₈₎]⁺), trihexyloctyl phosphonium(P₆₆₆₍₈₎]⁺), trihexyldecyl phosphonium (P₆₆₆₍₁₀₎]⁺), andtrihexyltetradecyl phosphonium (P₆₆₆₍₁₄₎]⁺).

In other more preferred embodiments, the cation of the lower viscosityionic liquid is [N(R³)(R⁴)(R⁵)(R⁶)]⁺, wherein R³, R⁴, R⁵ are selectedfrom C₄₋₁₄ alkyl, preferably C₆₋₁₀ alkyl, and R⁶ is selected from C₁₋₄alkyl, preferably C₁₋₂ alkyl. For example, the cation of the lowerviscosity ionic liquid may be selected from trioctylmethyl ammonium,tris(2-ethylhexyl) methyl ammonium, and tetrabutyl ammonium.

In other preferred embodiments, the cation of the lower viscosity ionicliquid is selected from imidazolium cations substituted with one or moreC₁₋₂₀ alkyl, C₃₋₈ cycloalkyl and C₆₋₁₀ aryl groups, preferablysubstituted with two C₁₋₁₀ alkyl groups, more preferably substitutedwith one methyl group and one C₁₋₁₀ alkyl group. For example, the cationof the lower viscosity ionic liquid may be selected from1-butyl-3-methyl imidazolium, 1-hexyl-3-methyl imidazolium and1-octyl-3-methyl imidazolium.

It will be understood that any suitable anionic group may be used as theanion of the lower viscosity ionic liquid. Preferably, the anion of thelower viscosity ionic liquid is as described previously in relation tothe anionic group [X⁻]. For example, it is most preferred that the anionof the lower viscosity ionic liquid is a non-coordinating anion asdescribed previously. It will be appreciated that there may be an excessof anions from the lower viscosity ionic liquid in comparison to theionic liquid extractant. Therefore, it is especially preferred that theanion of the lower viscosity ionic liquid is a non-coordinating anion.

For this reason, it is preferable to limit the total amount of halide orpseudohalide anions in the composition. For example, in preferredembodiments the composition comprises less than 25% halide orpseudohalide anions as a proportion of the total anions, preferably lessthan 20%, more preferably less than 15%, most preferably less than 10%,for example less than 5%. In some embodiments, the composition issubstantially free of halide or pseudohalide anions.

The composition may alternatively or additionally further comprise oneor more non-ionic liquid diluents. For example, in some preferredembodiments, the composition further comprises one or more organicsolvents. It will be understood that suitable organic solvents willinclude hydrophobic and non-coordinating solvents. The term“non-coordinating solvent” used herein, which is common in the field ofmetal coordination chemistry, is intended to mean a solvent that doesnot coordinate with metal atoms or ions, or does so only weakly.

Suitable organic solvents include but are not limited to hydrocarbonsolvents such as C₁₋₂₀ alkanes, alkenes or cycloalkanes, aromaticsolvents such as toluene or benzene, C₆₊ alcohols such as n-hexanol,etheric solvents such as diethyl ether, dipropyl ether, dibutyl etherand methyl-t-butyl ether, or halogenated solvents such astetrachloromethane, tetrachloroethane, chloroform, dichloromethane,chlorobenzene, or fluorobenzene. Preferably the organic solvent is ahydrocarbon solvent.

The ionic liquid may be present in the composition in any concentrationsuitable for extracting rare earth metals and it will be appreciatedthat this concentration will vary depending on the particularapplication and pH. In particular, it will be appreciated that for theseparation of rare earth metals a competitive separation is desirable.For example the concentration of the ionic liquid should be low enoughto avoid the extraction of all rare earth metals present. Therefore, theconcentration of the ionic liquid will typically depend on theconcentration of rare earth metals to be extracted and the pH at whichthe separation is conducted. In some preferred embodiments, the ionicliquid is present in the composition in a concentration of at least0.001 M, preferably from 0.005 M to 0.01 M.

In other embodiments, the composition may consist essentially of theionic liquid.

It will be appreciated that the concentration of the ionic liquid in thecomposition may be varied in order to achieve a particular targetviscosity for the composition. It will also be appreciated that thecharacter of the lower viscosity ionic liquid or other diluent may bevaried in order to obtain a particular viscosity level.

In preferred embodiments, the viscosity of the composition is in therange of from 50 to 500 mPa·s at 298K, when the composition comprises asolution of the ionic liquid in a lower viscosity ionic liquid. When theionic liquid is in a solution of an organic solvent, it will beappreciated that the composition will likely have a lower viscosity, forexample, less than 50 mPa·s. Viscosity may be measured by any suitablemethod, for example viscosity may be measured using a rotating diskviscometer with variable temperature.

The source of the rare earth metal is preferably a mineral or a wastematerial. In some embodiments, the acidic solution is obtainable byleaching the rare earth metal from its source using an acid, for examplea mineral acid such as hydrochloric, nitric, perchloric or sulfuricacid, typically hydrochloric or nitric acid. However, it will beappreciated that the acidic solution of the rare earth metal or mixtureof rare earth metals may be obtained in any suitable way from any rareearth metal source.

In preferred embodiments, the source of the rare earth metal source iscomminuted before leaching. This increases the surface area of the rareearth metal source that is exposed to leaching. Suitable methods forcomminuting the source of the rare earth metal include crushing,grinding or milling.

The concentration of rare earth metals in the acidic solution istypically from 60 ppm to 2000 ppm. Nonetheless, it will be appreciatedthat any suitable concentration of rare earth metals in the acidsolution may be used.

Typically, rare earth metals are obtained from rare earth ores, whichare mined and processed by a variety of methods depending on theparticular ore. Such processes are well known in the art. Usually,following mining such processes may include steps such as grinding,roasting to remove carbonates, chemical processing (e.g alkali/hydroxidetreatment), and ultimately leaching with acid to obtain an aqueousacidic solution containing a mixture of rare earth metals.

Examples of rare earth metal bearing minerals contained in rare earthores are aeschynite, allanite, apatite, bastnäsite, brannerite,britholite, eudialyte, euxenite, fergusonite, gadolinite, kainosite,loparite, monazite, parisite, perovskite, pyrochlore, xenotime,yttrocerite, huanghoite, cebaite, florencite, synchysite, samarskite,and knopite.

Rare earth metals may also increasingly be obtained from recycledmaterials. As global demand for rare earth metals grows, it isincreasingly attractive to obtain earth metals from recycled wastematerials, particularly in countries with a lack of minable rare earthore deposits. Rare earth waste materials may be obtained from varioussources, for example direct recycling of rare earth scrap/residues frompre-consumer manufacturing, “urban mining” of rare earth containing endof life products, or landfill mining of urban and industrial wastecontaining rare earths. As rare earth metals are increasingly being usedin consumer products, the amount of rare earth metals that can beobtained from such waste materials is also growing.

Waste materials that may contain rare earth metals include, magneticswarf and rejected magnets, rare earth containing residues from metalproduction/recycling (e.g. postsmelter and electric arc furnace residuesor industrial residues such as phosphogypsum and red mud), phosphorssuch as those in fluorescent lamps, LEDs, LCD backlights, plasma screensand cathode ray tubes, permanent magnets (e.g. NdFeB) such as those usedin automobiles, mobile phones, hard disk drives, computers andperipherals, electronic kitchen utensils, hand held tools, electricshavers, industrial electric motors, electric bicycles, electric vehicleand hybrid vehicle motors, wind turbine generators, nickel-metal hydridebatteries such as are used for rechargeable batteries and electric andhybrid vehicle batteries, glass polishing powders, fluid crackingcatalysts and optical glass. Major end-of-life waste material sources ofrare earths in terms of value are permanent magnets, nickel-metalhydride batteries and lamp phosphors, as well as scrap in the form ofmagnetic swarf waste.

Rare earth metals will usually be extracted from waste materials byleaching with mineral acids and optionally further processing to removeimpurities such as transition metals. This results in an acidic solutionof the rare earth metals, which may be used as a source for separationand purification of the individual rare earth metals.

Preferably the rare earth metal source is processed into an acidicsolution comprising rare earth metal by a process comprising:

-   -   i. dissolving the rare earth metal source in a first mineral        acid;    -   ii. adding a base to precipitate the rare earth metal as a salt;    -   iii. converting the salt into an oxide;    -   iv. converting the oxide into a halide salt; and    -   v. dissolving the halide salt in a second mineral acid to form        the acidic solution comprising rare earth metal.

This method is particularly suitable for rare earth metal sources thatare waste materials, such as metal magnet waste materials. Since thesesources typically contain a mixture of rare earth metals, then theproduct of step ii. may be a mixed rare earth metal salt, the product ofstep iii. may be a mixed rare earth metal oxide, the product of step iv.may be a mixed rare earth metal halide, and the product of step v. maybe an acidic solution comprising mixed rare earth metals (i.e. at leasta first and second rare earth metal). By carrying out thesepre-treatment steps, a purified mixed rare earth metal solution may beprovided.

The rare earth metal source is preferably comminuted, e.g. as describedabove, before step i. is carried out. Where the rare earth metal sourceis a magnet, it may also be demagnetised before step i., e.g. at atemperature of from 300 to 400° C. Demagnetization is preferably carriedout before the rare earth metal source is comminuted.

The mineral acid that is used in step i. is preferably sulfuric acid.Step i. may be carried out at elevated temperature (e.g. a temperatureof from 100 to 200° C.) and at elevated pressure (e.g. a pressure offrom 3 to 7 bar), though ambient conditions are also suitable.

A wide range of bases may be used in step ii. Suitable bases includeammonia, ammonium hydroxide and caustic soda. Step ii. may be carriedout at elevated temperature (e.g. a temperature of from 100 to 200° C.,or preferably a temperature of from 70 to 90° C., for example 75 to 85°C.) and at elevated pressure (e.g. a pressure of from 3 to 7 bar),though ambient conditions are also suitable. Since the acid-basereaction is exothermic, it is generally preferable to add the baseslowly in step ii. The precipitated salt that forms in step ii. may befiltered and washed before step iii. is carried out.

Steps i. and ii. may be carried out in the same vessel, preferably astirred vessel.

Step iii. may be carried out by

-   -   iiia. contacting the salt with oxalic acid to give an oxalate        salt; and    -   iiib. converting, e.g. by calcination, the oxalate salt into the        rare earth metal oxide.

Step iiia may be carried out under ambient conditions (e.g. at atemperature of from 15 to 30° C., and at a pressure of approximately 100kPa). The oxalate salt may be filtered and washed before step iiib. iscarried out. Step iiib. may be carried out at a temperature of from 900to 1100° C., and at ambient pressure (e.g. at a pressure ofapproximately 100 kPa). The oxide may be filtered and washed before stepiv. is carried out.

Step iv. is preferably carried out by contacting the oxide with ahydrogen halide acid, and more preferably with hydrochloride acid. Stepiv. may be carried out at elevated temperature (e.g. at a temperature offrom 100 to 200° C.) and at elevated pressure, though ambient conditionsare also suitable. The halide salt may be further purified before stepv. is carried out, e.g. by recrystallisation. However, it will often besufficient to simply wash the halide salt with water or the hydrogenhalide acid.

Step v. is preferably carried out by dissolving the halide salt in ahydrogen halide acid, preferably hydrochloride acid.

Thus, it is an advantage of the present invention that rare earth metalsmay be extracted with high selectivity and extractability directly froman acidic solution of the rare earth metal, which may be convenientlyobtained from the extraction process of an ore or a waste material.

The ionic liquid defined herein may be prepared by a method whichcomprises reacting:

-   -   where: LG represents a leaving group.

A “leaving group” as used herein will be understood to mean a group thatmay be displaced from a molecule by reaction with a nucleophilic centre,in particular a leaving group will depart with a pair of electrons inheterolytic bond cleavage. A leaving group is usually one that is ableto stabilize the additional electron density that results from bondheterolysis. Such groups are well-known in the field of chemistry.

It will be understood that the group [Z] may be any group that is ableto displace the leaving group to form a [Z⁺] cation as definedpreviously herein.

It will be appreciated that a leaving group as defined herein will besuch that the primary amine coupled by L₁ to [Z] may displace theleaving group to form a bond between the nitrogen and an L₂ group, andsuch that the group [Z] can displace the leaving group to form a bondbetween [Z] and an L₂ group.

Leaving groups may, for example, include a group selected fromdinitrogen, dialkyl ethers, perfluoroalkylsulfonates such as triflate,tosylate or mesylate, halogens such as Cl, Br and I, water, alcohols,nitrate, phosphate, thioethers and amines. Preferably, the leaving groupLG is selected from halides, more preferably the leaving group LG is Cl.

Such substitution reactions as described herein are well-known in theart and could be performed by a skilled person without difficulty.

By preparing an ionic liquid by this method, an ionic liquid havingadvantageous rare earth metal extraction properties may be convenientlysynthesised in a single step, reducing the increased costs associatedwith multiple step syntheses.

The present invention will now be illustrated by way of the followingexamples and with reference to the following figures in which:

FIG. 1 is a graph showing the distribution factors for the extraction ofa selection of rare earth metals according to an embodiment of thepresent invention; and

FIG. 2 shows the crystal structure of the [MAIL]⁺ cation coordinating toNd after extraction from an acidic (HCl) solution containing NdCl₃.6H₂O.

FIG. 3 is a graph showing extraction of a selection of rare earth metalsusing [MAIL⁺][R₂P(O)O⁻].

FIG. 4 is a graph showing extraction of a selection of rare earth metalsusing [MAIL-6C⁺][NTf₂ ⁻].

FIG. 5 is a graph showing extraction of a selection of rare earth metalsusing [MAIL-Ph⁺][NTf₂ ⁻].

EXAMPLES Example 1: Synthesis of Ionic Liquid

General Procedure for the Synthesis of an Ionic Liquid According toEmbodiments of the Invention

A reaction mixture comprising 3 moles of anN,N-dialkyl-2-chloroacetamide and a substrate having the structureH₂N-L₁-[Z] were stirred in a halogenated solvent (e.g. CHCl₃, CH₂Cl₂,etc.) or an aromatic solvent (e.g. toluene, xylene, etc.) at 60 to 70°C. for 7 to 15 days. After cooling, the solid was filtered off and theorganic phase was repeatedly washed with 0.1 to 0.2 M HCl until theaqueous phase showed milder acidity (pH≥2). The organic phase was thenwashed with 0.1 M Na₂CO₃ (2-3 washes) and finally was washed withdeionized water until the aqueous phase showed a neutral pH. The solventwas removed under high vacuum to give the ionic liquid product (with achloride anion) as a highly viscous liquid. This ionic liquid could beused as it was or the chloride anion could be exchanged with differentanions (e.g. bistriflimide, triflate, hexafluorophosphate etc.) usingconventional metathesis routes, for example, by reacting with an alkalimetal salt of the desired anion with the ionic liquid in an organicsolvent.

Synthesis of an Imidazolium Ionic Liquid

[MAIL⁺][NTf₂ ⁻]:

1-(3-Aminopropyl)-imidazole (0.05 mol) was added to ofN,N-diisobutyl-2-chloroacetamide (0.15 mol) in a 500 ml three neckedround bottom flask. Triethylamine (0.11 moles) was then added along withchloroform (200 ml). The reaction was stirred for 6 hours at roomtemperature and then stirred at 60 to 70° C. for 7 days. The reactionmixture was then cooled and after filtration it was successively washedwith 0.1 M HCl, 0.1 M Na₂CO₃ and deionized water (as described ingeneral procedure). The solvent was removed from the neutralised organicphase at 8 mbar (6 mm Hg) and finally at 60° C. and 0.067 mbar (0.05mmHg). The ionic liquid [MAIL⁺]Cl⁻ was recovered as a highly viscousyellow liquid.

Ionic liquid [MAIL⁺]Cl⁻ (0.025 mol) was dissolved in chloroform andlithium bis-(trifluoromethane) sulfonamide (LiNTf₂) (0.03 mol) wasadded. The reaction mixture was stirred for 1 hour and then the organicphase was repeatedly washed with deionized water. Finally the solventwas removed from the organic phase under vacuum (0.13 mbar, 0.1 mm Hg)at 65° C. to yield the bistriflimide anion form of the ionic liquid([MAIL⁺][NTf₂ ⁻]). [MAIL-6C⁺][NTf₂ ⁻]:

A mixture of potassium pthalimide (10.0 g, 54.0 mmol) and1,6-dibromobutane (9.91 mL, 64.8 mmol) in dry DMF (100 mL) was stirredat room temperature for 12 days. The mixture was concentrated andextracted with chloroform (3×30 mL) and washed with deionised water(3×80 mL) and brine (100 mL). The organic layer was dried over magnesiumsulfate and concentrated to give a white syrup. The syrup was trituratedwith hexanes, filtered and dried to give a white solid product (3) (14.3g, 85%).

To NaH (0.645 g, 26.9 mmol) in THF was added at 0° C. under N₂,imidazole (1.21 g, 17.7 mmol) in THF was added over 30 mins, and stirredfor a further 30 mins at 0° C. 3 (5.00 g. 16.1 mmol) in THF was added at0° C. and the mixture stirred for 1 hour at room temperature, thenrefluxed at 70° C. overnight. The mixture was filtered and the residualNaBr was washed with THF. The filtrate was concentrated to give a syrupwhich was dissolved in DCM to give a yellow solution which was thenwashed with water and dried over sodium sulfate and triturated withhexanes to precipitate a white solid which was filtered and washed withhexanes (4) (1.52 g, 32%).

4 (0.750 g, 2.54 mmol) was dissolved in a EtOH:H₂O mixture (160 mL, 3:1)and hydrazine hydrate (50-60%, 0.174 mL, 5.55 mmol) was added at roomtemperature and the mixture refluxed overnight. The solution was cooledto room temperature and concentrated HCl (2 mL) was added, the reactionmixture changed from colourless to yellow to red to light yellow duringthe addition. The mixture was stirred at reflux for 6 hours andfiltered. The solution was concentrated and dissolved in distilled waterto give a yellow solution. Sodium hydroxide was added until the mixturereached pH 11, it was then extracted with chloroform (4×40 mL), driedover magnesium sulfate and concentrated to give an orange oil (5) (0.329g, 78%).

To a high pressure vessel was added 5 (0.257 g, 1.54 mmol),triethylamine (0.623 g, 6.16 mmol), N,N-diisobutyl-2-chloroacetamide(0.950 g, 4.62 mmol) and chloroform (5 mL). The vessel was stoppered andstirred at 140° C. on an oil bath for 16 hours. The reaction mixture waswashed with pH 1 HCl (40 mL), Na₂CO₃ (2×40 mL) then water (4×40 mL). Theorganic layer was dried over magnesium sulfate and concentrated in vacuoto give a viscous dark brown liquid (6) (0.648 g, 59%).

To a round bottom flask was added 6 (0.6255 g, 0.88 mmol) followed byDCM (50 mL). LiNTf₂ (0.7572 g, 2.64 mmol) was added followed by water(50 mL). The reaction mixture was stirred at room temperature for 24hours. The aqueous layer was removed and the organic layer washed withdeionised water (4×40 mL). The organic layer was dried over magnesiumsulfate and concentrated. The product was dried overnight to give ablack viscous liquid, [MAIL-6C⁺][NTf₂ ⁻], (0.7467 g, 89%).

[MAIL-Ph⁺][NTf₂ ⁻]:

To a high pressure vessel was added 1-(3-aminopropyl)imidazole (0.200 g,1.60 mmol), triethylamine (0.647 g, 6.39 mmol),2-chloro-N,N-diphenylacetamide (1.18 g, 4.49 mmol) and chloroform (5mL). The vessel was stoppered and stirred at 145° C. on an oil bath for16 hours. The reaction mixture was washed with pH 1 HCl (15 mL), thenwater (4×150 mL). The organic layer was dried over magnesium sulfate andconcentrated in vacuo to give an orange/brown solid (7) (0.883 g, 70%).

To a 50 mL round-bottom flask was added 7 (0.444 g, 0.560 mmol) followedby DCM (20 mL). LiNTf₂ (0.484 g, 1.69 mmol) was added followed bydeionised water (20 mL). The reaction mixture was stirred at roomtemperature for 24 hours. The aqueous layer was removed and the organiclayer washed with deionised water (5×15 mL). The organic layer was driedover magnesium sulfate and concentrated. The product was dried overnightto give a viscous brown liquid, [MAIL-Ph⁺][NTf₂ ⁻], (0.351 g, 65%).

The phosphinate ionic liquid [MAIL⁺][R₂P(O)O⁻] (R=2,4,4-trimethylpentyl)was also synthesised by ion exchange.

Synthesis of Phosphonium Ionic Liquids

[MAIL-PPh₃ ⁺][NTf₂ ⁻]:

To a 50 mL round-bottom flask equipped with a magnetic stir bartriphenylphosphine (0.836 g, 3.19 mmol), 3-bromopropylamine hydrobromide(1.00 g, 4.57 mmol), and acetonitrile (25 mL) were added. The suspensionwas then heated and stirred at reflux for 16 hours. The reaction wascooled to room temperature, and the solvent was removed under reducedpressure, and the resulting white solid was then dried in vacuo, andused in subsequent steps without further purification (1.01 g, 79%).

To a 50 mL round-bottom flask was added(3-Aminopropyl)(triphenyl)phosphonium bromide (1.01 g, 0.252 mmol)followed by DCM (20 mL). LiNTf₂ (2.17 g, 7.55 mmol) was added followedby deionised water 20 mL). The reaction mixture was stirred at roomtemperature for 24 hours. The aqueous layer was removed and the organiclayer washed with deionised water (5×15 mL). The organic layer was driedover magnesium sulfate and concentrated. The product was dried overnightto give a white solid (1.26 g, 84%).

To a high pressure vessel was added(3-Aminopropyl)(triphenyl)phosphonium bistriflimide (0.200 g, 0.333mmol), triethylamine (0.135 g, 1.33 mmol),N,N-diisobutyl-2-chloroacetamide (0.137 g, 0.666 mmol) and chloroform (5mL). The vessel was stoppered and stirred at 145° C. on an oil bath for48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), thenwater (4×150 mL). The organic layer was dried over magnesium sulfate andconcentrated in vacuo to give a viscous dark brown liquid, [MAIL-PPh₃⁺][NTf₂ ⁻], (0.282 g, 90%).

[MAIL-PPh₃ ⁺][R₂P(O)O⁻]:

To a high pressure vessel was added(3-Aminopropyl)(triphenyl)phosphonium bromide (1.01 g, 2.53 mmol),triethylamine (1.03 g, 10.1 mmol), N,N-diisobutyl-2-chloroacetamide(1.04 g, 5.07 mmol) and chloroform (5 mL). The vessel was stoppered andstirred at 145° C. on an oil bath for 48 hours. The reaction mixture waswashed with pH 1 HCl (15 mL), then water (4×150 mL). The organic layerwas dried over magnesium sulfate and concentrated in vacuo to give aviscous dark brown liquid (0.981 g, 56%).

To a 50 mL round-bottom flask was added the phosphonium diamide (0.898g, 1.29 mmol) followed by DCM (20 mL). R₂P(O)OH(R=2,4,4-trimethylpentyl) (0.356 g, 1.29 mmol) was added followed by aKOH solution (40%, 20 mL). The reaction mixture was stirred at 50° C.for 16 hours. The aqueous layer was removed and the organic layer washedwith deionised water (5×15 mL). The organic layer was dried overmagnesium sulfate and concentrated. The product was dried overnight togive a white solid, [MAIL-PPh₃ ⁺][R₂P(O)O⁻], (0.943 g, 77%).

[MAIL-P₄₄₄ ⁺][NTf₂ ⁻]:

To a 50 mL round-bottom flask equipped with a magnetic stir bartributylphosphine (0.823 g, 4.07 mmol), 3-bromopropylamine hydrobromide(0.890 g, 4.07 mmol), and acetonitrile (25 mL) were added. Thesuspension was then heated and stirred at reflux for 48 hours. Thereaction was cooled to room temperature, and the solvent was removedunder reduced pressure, and the resulting oil was then dried in vacuo,and used in subsequent steps without further purification (1.24 g, 89%).

To a 50 mL round-bottom flask was added(3-Aminopropyl)(tributyl)phosphonium bromide (0.559 g, 1.64 mmol)followed by DCM (20 mL). LiNTf₂ (1.41 g, 4.93 mmol) was added followedby deionised water (20 mL). The reaction mixture was stirred at roomtemperature for 24 hours. The aqueous layer was removed and the organiclayer washed with deionised water (5×15 mL). The organic layer was driedover magnesium sulfate and concentrated. The product was dried overnightto give a colourless oil (0.304 g, 34%).

To a high pressure vessel was added (3-Aminopropyl)(tributyl)phosphoniumbistriflimide (0.200 g, 0.370 mmol), triethylamine (0.150 g, 1.48 mmol),N,N-diisobutyl-2-chloroacetamide (0.152 g, 0.740 mmol) and chloroform (5mL). The vessel was stoppered and stirred at 145° C. on an oil bath for48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), thenwater (4×150 mL). The organic layer was dried over magnesium sulfate andconcentrated in vacuo to give a viscous dark brown liquid, [MAIL-P₄₄₄⁺][NTf₂ ⁻], (0.250 g, 77%).

[MAIL-P₈₈₈ ⁺][NTf₂ ⁻]:

To a 50 mL round-bottom flask equipped with a magnetic stir bartrioctylphosphine (0.872 g, 2.35 mmol), 3-bromopropylamine hydrobromide(0.500 g, 2.28 mmol), and acetonitrile (25 mL) were added. Thesuspension was then heated and stirred at reflux for 48 hours. Thereaction was cooled to room temperature, and the solvent was removedunder reduced pressure, and the resulting oil was then dried in vacuo,and used in subsequent steps without further purification (0.889 g,85%).

To a 50 mL round-bottom flask was added(3-Aminopropyl)(trioctyl)phosphonium bromide (0.564 g, 1.11 mmol)followed by DCM (20 mL). LiNTf₂ (0.954 g, 3.32 mmol) was added followedby deionised water (20 mL). The reaction mixture was stirred at roomtemperature for 24 hours. The aqueous layer was removed and the organiclayer washed with deionised water (5×15 mL). The organic layer was driedover magnesium sulfate and concentrated. The product was dried overnightto give a colourless oil (0.542 g, 69%).

To a high pressure vessel was added (3-Aminopropyl)(trioctyl)phosphoniumbistriflimide (0.200 g, 0.282 mmol), triethylamine (0.114 g, 1.13 mmol),N,N-diisobutyl-2-chloroacetamide (0.116 g, 0.564 mmol) and chloroform (5mL). The vessel was stoppered and stirred at 145° C. on an oil bath for48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), thenwater (4×150 mL). The organic layer was dried over magnesium sulfate andconcentrated in vacuo to give a viscous dark brown liquid, [MAIL-P₈₈₈⁺][NTf₂ ⁻]0.313 g, 99%).

Example 2: Liquid-Liquid Extraction of Rare Earth Metals Using[MAIL⁺][NTf₂ ⁻]

General Procedure for Extraction of Rare Earth Metals

Equal volumes (2 to 5 ml) of the ionic liquid extractant ([MAIL⁺][NTf₂⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻]) and an acidic aqueous feed solutioncontaining rare earth metals in HCl were equilibrated for 15-30 minuteson a wrist action shaker. The phases were centrifuged and the aqueousphase was analysed for rare earth metal content using InductivelyCoupled Plasma Optical Emission Spectroscopy (ICP-OES), though it willbe appreciated that any suitable analysis technique may be used. Theproportion of the rare earth metals extracted into the ionic liquid(organic) phase was determined through mass balance using the ICP-OESmeasurement.

The distribution ratio of an individual rare earth metal was determinedas the ratio of its concentration in the ionic liquid phase to that ofit in the aqueous phase (raffinate). D_(M)=[M]_(IL)/[M]_(Aq), where ILrepresents ionic liquid phase and Aq represents the aqueous phase(raffinate).

The separation factor (SF) with respect to an individual rare earthmetal pair is expressed as the ratio of the distribution ratio of afirst rare earth metal with the distribution ratio of a second rareearth metal. For example, the separation factor of dysprosium withrespect to neodymium=D_(Dy)/D_(Nd). It will be appreciated thatseparation factors estimated from independently obtained distributionratios will be lower than the actual separation factors, obtained duringthe separation of mixtures of rare earth metals during a competitiveseparation (as exemplified below).

Distribution ratios for individual rare earth metals were obtained inseparate extractions according to the general procedure above, using0.0075 M [MAIL⁺][NTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻] and a 200 mg/l (ppm)HCl solution of the relevant rare earth metal chloride (where 200 ppmrefers to the concentration of the elemental metal in the solution).FIG. 1 shows a plot of the distribution ratios for each rare earth metalas a function of pH, showing that the ionic liquid according to thepresent invention may be used to extract rare earth metals across arange of pH values.

The separation of rare earth metals was also performed by the abovemethod using 0.0075 M of the ionic liquids [MAIL⁺][R₂P(O)O⁻],[MAIL-6C⁺][NTf₂ ⁻] and [MAIL-Ph⁺][NTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻]. Theseionic liquids were also found to differentially extract rare earthmetals at pH 1 to pH4 as shown in FIGS. 3, 4 and 5.

Recycling of Ionic Liquid

Dy was extracted from an aqueous solution of Dy (180 ppm) at pH4 using0.025 M [MAIL⁺][NTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻] (>95% extracted) and theionic liquid stripped at pH 1 using HCl (1:1 ionic liquid to strippingsolution ratio) in 4 contacts. The ionic liquid was washed withdeionised water to raise the pH to 7, and was used in furtherextractions. The amount of Dy extracted dropped by around 20% comparedto the first extraction, but remained at a constant level over foursubsequent extractions.

Separation of Dy and Nd

An aqueous HCl solution containing DyCl₃.6H₂O (60 mg/l (ppm) Dy) andNdCl₃.6H₂O (1400 mg/l (ppm) Nd) at pH 3 was extracted with the ionicliquid extractant (0.005 M [MAIL⁺][INTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][INTf₂ ⁻])according to the general procedure above. A single contact (extraction)gave D_(Dy)=13.45, D_(Nd)=0.0124, giving a SF_(Dy-Nd) of 1085.

This separation factor (1085) is considerably higher than the separationfactors obtained for Dy/Nd separation by the systems in the prior artshown in Table 1 (maximum 239).

The above separation was repeated using 0.0075M of an ionic liquid in[P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻] at pH2. The extraction was performed using[MAIL⁺][NTf₂ ⁻], [MAIL⁺][R₂P(O)O⁻], [MAIL-6C⁺][NTf₂ ⁻], [MAIL-P₄₄₄⁺][NTf₂ ⁻], [MAIL-P₈₈₈ ⁺][NTf₂ ⁻], [MAIL-PPh₃ ⁺][NTf₂ ⁻] and [MAIL-PPh₃⁺][R₂P(O)O⁻] and the results are shown in Table 2. As can be seen, ionicliquids described herein can be used to completely selectively extractDy from Nd. Completely selective extraction of Dy from Nd using[MAIL⁺][NTf₂ ⁻], [MAIL⁺][R₂P(O)O⁻] and [MAIL-6C⁺][NTf₂ ⁻] was alsoobserved at pH 1.8, with extraction of more than 50% Dy.

TABLE 2 Ionic liquid Dy % Extraction Nd % Extraction [MAIL⁺][NTf₂ ⁻] 820 [MAIL⁺][R₂P(O)O⁻] 86.5 0 [MAIL-6C⁺][NTf₂ ⁻] 83 0 [MAIL-P₄₄₄ ⁺][NTf₂ ⁻]89 0 [MAIL-P₈₈₈ ⁺][NTf₂ ⁻] 87 0 [MAIL-PPh₃ ⁺][NTf₂ ⁻] 90 0.6 [MAIL-PPh₃⁺][R₂P(O)O⁻] 90 0

Separation of Eu and La

An aqueous HCl solution containing EuCl₃.6H₂O (65 mg/l (ppm) Eu) andLaCl₃.7H₂O (470 mg/l (ppm) La) at pH 3 was extracted with the ionicliquid extractant (0.005 M [MAIL⁺][NTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻])according to the general procedure above. A single contact (extraction)gave D_(Eu)=9.3, D_(La)=0.044, giving a SF_(Eu-La) of 211.

Separation of Tb and Ce

An aqueous HCl solution containing TbCl₃.6H₂O (530 mg/l (ppm) Tb) andCeCl₃.6H₂O (950 mg/l (ppm) Ce) at pH 3 was extracted with the ionicliquid extractant (0.0075 M [MAIL⁺][NTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻])according to the general procedure above. A single contact (extraction)gave D_(Tb)=11.2, D_(Ce)=0.068, giving a SF_(Tb—Ce) of 162.

Example 3: Stripping of Rare Earth Metals from [MAIL⁺][NTf₂ ⁻]

Dy(III) (80 ppm) was stripped from an organic phase at pH 0.25comprising [MAIL⁺][NTf₂ ⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻] (0.0075 M) in 3successive contacts. The organic phase was contacted with an equalvolume of an aqueous HCl solution (0.55 M) and was equilibrated for15-30 minutes on a wrist action shaker. 67 ppm of Dy(III) was strippedin the first contact, 10 ppm was stripped in the second contact, and 2ppm was stripped in the third contact.

Similarly, from observation of the distribution ratios in FIG. 1, it isclear that heavy rare earth metals such as Tm, Yb and Lu havesignificantly reduced distribution factors with increasing acidity.Thus, it is also expected that heavy rare earth metals may be strippedfrom the ionic liquid of the present invention at relatively high pHvalues.

The above examples show that a large increase in the separation factorsbetween key rare earth metal pairs may be obtained by use of an ionicliquid according to the present invention (e.g. Nd/Dy: Nd—Dy magnet,Eu/La: white lamp phosphor, Tb/Ce: green lamp phosphor). The rare earthmetals may also be advantageously stripped from the ionic liquid atrelatively high pH compared to prior art systems.

Without wishing to be bound by any particular theory, it is believedthat a more pronounced increase in distribution ratios is observed forheavier rare earth metals than lighter rare earth metals as a result ofincreased formation of the more hydrophobic doubly coordinated rareearth metal species M.([MAIL⁺][NTf₂ ⁻])₂ over the singly coordinatedspecies M.([MAIL⁺][NTf₂ ⁻]). It is believed that the more hydrophobicspecies will be more easily extracted into the organic phase duringseparation, leading to increased distribution ratios.

Nuclear magnetic resonance, infra-red and mass spectrometry studies haveshown that the doubly coordinated species is more abundant in solutionsof Lu and the ionic liquid compared to solutions of La and the ionicliquid, highlighting the differentiation between the heavy and lightrare earth metals achieved by the ionic liquid of the present invention.

Furthermore, optimised geometries of the complexes LaCl₃.([MAIL⁺][Cl⁻])₂and LuCl₃.([MAIL⁺][Cl⁻])₂ show that the distance between the tertiarycentral nitrogen of the ionic liquid cation and the metal is much longerin the case of La (˜2.9 Å, non-bonding) than in the case of Lu (˜2.6 Å,bonding), which also supports the weaker bonding of the ionic liquid tolighter rare earth metals. At the same time, the electron donatinggroups, in this case amides, linked to the nitrogen atom bond to themetal in a very similar way in both cases. This result shows that thecentral motif of the ionic liquid cation having a tertiary nitrogendonor is important for the differentiation obtained between the heavierand lighter rare earth metals and the improved selectivity that resultstherefrom.

Example 4: Preparation of Rare Earth Metal Oxides

An aqueous HCl stripping solution composition comprising Dy is contactedwith oxalic acid at room temperature and pressure. The mixture isstirred. Dysprosium oxalate (Dy₂(C₂O₄)₂) precipitates from the solution.The precipitate is separated from the acidic stripping solution byfiltration, and is washed with water. The dysprosium oxalate precipitateis calcined at 1000° C. to give dysprosium oxide (Dy₂O₃).

Example 5: Extraction of Rare Earth Metals from a Magnet Sample

A magnet sample containing rare earth metals was obtained in powderedform and was converted to the chloride form as follows. The magnet feedwas dissolved in 2 M H₂SO₄. The undissolved impurities were removed byfiltration. The pH was raised to 1.5 using ammonium hydroxide at 60° C.At 60° C. the rare-earth sulphates crash out of solution leaving theiron sulphate impurity in solution. The separated rare-earth sulphatewas converted to the oxalate (by contacting with oxalic acid to andwashing the rare-earth oxalate with water) and calcined at 900° C. toform the rare-earth oxide. The rare-earth oxide is converted into therare-earth chloride by leaching into a HCl solution and recrystallised.

A feed solution of 0.2 g rare-earth chloride salt in 50 mL pH 2 solution(HCl) was prepared. The feed solution had an initial concentration of20.93 ppm Dy and 1573.81 ppm Nd.

Separate extractions were carried out as described in Example 2, using0.0075 M [MAIL⁺][NTf₂ ⁻] or [MAIL⁺][R₂P(O)O⁻] in [P₆₆₆₍₁₄₎ ⁺][NTf₂ ⁻] atpH 2. The ionic liquids were both found to extract more than 90% of theDy in the solution after 4 contacts, whilst extracting less than 5% Nd.

The invention claimed is:
 1. A method for preparing a rare earth metaloxide from a mixture of rare earth metals, said method comprising:contacting an acidic solution of the mixture of rare earth metals with acomposition which comprises an ionic liquid to form an aqueous phase anda non-aqueous phase into which a rare earth metal has been selectivelyextracted; recovering the rare earth metal from the non-aqueous phase;and processing the recovered rare earth metal into a rare earth metaloxide, wherein the ionic liquid has the formula:[Cat⁺][X⁻] in which: [Cat⁺] represents a cationic species having thestructure:

where: [Y⁺] comprises ammonium, benzimidazolium, benzofuranium,benzothiophenium, benzotriazolium, borolium, cinnolinium,diazabicyclodecenium, diazabicyclononenium,1,4-diazabicyclo[2.2.2]octanium, diazabicyclo-undecenium, dithiazolium,furanium, guanidinium, imidazolium, indazolium, indolinium, indolium,morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium,iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium,piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium,pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium,quinazolinium, quinolinium, iso-quinolinium, quinoxalinium,quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium,iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium,thiuronium, triazinium, triazolium, iso-triazolium or uronium; each EDGrepresents an electron donating group; L₁ is selected from C₁₋₁₀alkanediyl, C₂₋₁₀ alkenediyl, C₁₋₁₀ dialkanylether or C₁₋₁₀dialkanylketone; each L₂ is independently selected from C₁₋₂ alkanediyl,C₂ alkenediyl, C₁₋₂ dialkanylether or C₁₋₂ dialkanylketone; and [X⁻]represents an anionic species.
 2. The method of claim 1, wherein therare earth metal is recovered from the non-aqueous phase by strippingwith an acidic stripping solution to produce an acidic strippingsolution comprising the recovered rare earth metal.
 3. The method ofclaim 2, wherein processing the recovered rare earth metal into a rareearth metal oxide comprises: contacting the acidic stripping solutioncomprising the recovered rare earth metal with oxalic acid to give arare earth metal oxalate; and converting, by calcination, the rare earthmetal oxalate into the rare earth metal oxide.
 4. The method of claim 3,wherein the rare earth metal oxalate is calcined at a temperature of atleast 500° C.
 5. The method of claim 1, wherein the method furthercomprises processing the rare earth metal oxide into a magnet.
 6. Themethod of claim 1, wherein the acidic solution comprises a first and asecond rare earth metal, and the method comprises: (a) partitioning thefirst rare earth metal into the non-aqueous phase, recovering the firstrare earth metal from the non-aqueous phase, and processing therecovered first rare earth metal into a rare earth metal oxide.
 7. Themethod of claim 6, wherein the method further comprises, in step (a),separating the non-aqueous phase from the acidic solution; and (b)contacting the acidic solution depleted of the first rare earth metalwith the composition which comprises an ionic liquid.
 8. The method ofclaim 6, wherein: the first rare earth metal is dysprosium, and thesecond rare earth metal is neodymium; or the first rare earth metal iseuropium, and the second rare earth metal is lanthanum.
 9. The method ofclaim 1, wherein: the acidic solution from which the rare earth metal isextracted has a pH of from 2 to 4; the composition is added to theacidic solution in a volume ratio of from 0.5:1 to 2:1; prior tocontacting the composition with the acidic solution of the rare earthmetal the composition is equilibrated with an acidic solution having thesame pH as the acidic solution of the rare earth metal; the acidicsolution is contacted with the composition for from 1 to 40 minutes;and/or the method comprises contacting and physically mixing the acidicsolution of the rare earth metal and the composition.
 10. The method ofclaim 1, wherein when the nitrogen linking L₁ to each L₂ and one of theEDG both coordinate to a metal, the ring formed by the nitrogen, L₂, theEDG and the metal is a 5 or 6 membered ring.
 11. The method of claim 1,wherein [Y⁺] represents: an acyclic cation selected from:[—N(R^(a))(R^(b))(R^(c))]⁺, [—P(R^(a))(R^(b))(R^(c))]⁺ and[—S(R^(a))(R^(b))]⁺, where: R^(a), R^(b) and R^(c) are eachindependently selected from optionally substituted C₁₋₃₀ alkyl, C₃₋₈cycloalkyl and C₆₋₁₀ aryl groups; or a cyclic cation selected from:

where: R^(a), R^(b), R^(c), R^(d), R^(e) and R^(f) are eachindependently selected from: hydrogen and optionally substituted C₁₋₃₀alkyl, C₃₋₈ cycloalkyl and C₆₋₁₀ aryl groups, or any two of R^(a),R^(b), R^(c), R^(d) and R^(e) attached to adjacent carbon atoms form anoptionally substituted methylene chain —(CH₂)_(q)— where q is from 3 to6; or a saturated heterocyclic cation having the formula:

where: R^(a), R^(b), R^(c), R^(d), R^(e) and R^(f) are eachindependently selected from: hydrogen and optionally substituted C₁₋₃₀alkyl, C₃₋₈ cycloalkyl and C₆₋₁₀ aryl groups, or any two of R^(a),R^(b), R^(c), R^(d) and R^(e) attached to adjacent carbon atoms form anoptionally substituted methylene chain —(CH₂)_(q)— where q is from 3 to6.
 12. The method of claim 11, wherein [Y⁺] represents a cyclic cationselected from:

wherein R^(f) is a substituted C₁₋₅ alkyl group, and the remainder ofR^(a), R^(b), R^(c), R^(d), R^(e) and R^(f) are independently selectedfrom H and unsubstituted C₁₋₅ alkyl groups.
 13. The method of claim 1,wherein L₁ represents: a linking group selected from C₁₋₁₀ alkanediyland C₁₋₁₀ alkenediyl groups.
 14. The method of claim 1, wherein each L₂represents: a linking group independently selected from C₁₋₂ alkanediyland C₂ alkenediyl groups.
 15. The method of claim 1, wherein each EDGrepresents: an electron donating group independently selected from—CO₂R^(x), —OC(O)R^(x), —CS₂R^(x), —SC(S)R^(x), —S(O)OR^(x),—OS(O)R^(x), —NR^(x)C(O)NR^(y)R^(z), —NR^(x)C(O)OR^(y),—OC(O)NR^(y)R^(z), —NR^(x)C(S)OR^(y), —OC(S)NR^(y)R^(z),—NR^(x)C(S)SR^(y), —SC(S)NR^(y)R^(z), —NR^(x)C(S)NR^(y)R^(z),—C(O)NR^(y)R^(z), —C(S)NR^(y)R^(z), wherein R^(x), R^(y) and R^(z) areindependently selected from H or C₁₋₆ alkyl.
 16. The method of claim 1,wherein [Cat⁺] represents one or more ionic species having thestructure:

where: [Z⁺] is selected from ammonium, benzimidazolium, benzofuranium,benzothiophenium, benzotriazolium, borolium, cinnolinium,diazabicyclodecenium, diazabicyclononenium,1,4-diazabicyclo[2.2.2]octanium, diazabicyclo-undecenium, dithiazolium,furanium, guanidinium, imidazolium, indazolium, indolinium, indolium,morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium,iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium,piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium,pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium,quinazolinium, quinolinium, iso-quinolinium, quinoxalinium,quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium,iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium,thiuronium, triazinium, triazolium, iso-triazolium or uronium.
 17. Themethod of claim 1, wherein [X⁻] represents one or more anionic speciesselected from: hydroxides, halides, perhalides, sulphates, sulphites,sulfonates, sulfonimides, phosphates, phosphites, phosphonates,phosphinates, methides, borates, carboxylates, azolates, carbonates,carbamates, thiophosphates, thiocarboxylates, thiocarbamates,thiocarbonates, xanthates, thiosulfonates, thiosulfates, nitrate,nitrite, tetrafluoroborate, hexafluorophosphate, perchlorate,halometallates, amino acids, borates and polyfluoroalkoxyaluminates. 18.The method of claim 1, wherein the composition further comprises a lowerviscosity ionic liquid and/or one or more organic solvents; and/or theanion of the ionic liquid is present in the composition in aconcentration of at least 0.001 M.
 19. The method of claim 1, whereinthe method comprises preparing the acidic solution of rare earth metalby leaching the rare earth metal from its source with an acid.
 20. Themethod of claim 19, wherein the method further comprising preparing theacidic solution of rare earth metal by a process which comprises:dissolving the rare earth metal source in a first mineral acid; adding abase to precipitate the rare earth metal as a salt; converting the saltinto an oxide; converting the oxide into a halide salt; and dissolvingthe halide salt in a second mineral acid to form the acidic solutioncomprising rare earth metal.